In order to reduce the voltage necessary for starting the discharge in metal vapor arc lamps, there is generally included an inert starting gas at a relatively low pressure. For instance, in the case of mercury vapor lamps and metal halide lamps which also contain mercury, the starting gas commonly used in commercially available lamps is argon at a pressure from 20 to 40 torr.
In the miniature metal vapor lamps with which the invention is particularly concerned, the small internal surface area of the arc tube entails rapid blackening should there be any electrode sputtering during operation of the lamp. Sputtering tends to occur at starting during the glow to arc transition (GAT) phase and thus it becomes important to shorten as much as possible the duration of the GAT. It is well-known to shorten the GAT by increasing the fill pressure of the starting gas but this also causes the starting voltage to increase. For instance a miniature metal halide lamp using argon for the starting gas at a fill pressure of 60 torr has a starting voltage in excess of 600 volts. The small arc tube blackens too rapidly with the result that the lamp has poor lumen maintenance. Increasing the fill pressure to 100 torr reduces the blackening but causes the starting voltage to increase to about 700 volts. In order to adequately suppress arc tube blackening due to sputtering of electrodes during lamp starting, the fill pressure of the starting gas should be increased into the range of 100 to 200 torr. However the starting voltage for a lamp with such a high argon pressure would be about 1,000 volts and this of course means that a high cost ballast would be required to start and operate the lamp.